1. Field of the Invention
The present invention relates to a multi-stage transmission for a vehicle such as tractors.
2. Description of the Related Art
A weight of a tractor-trailer greatly changes from when it runs with a trailer to when without a trailer. In order to improve vehicle""s running performance, a splitter (secondary transmission) having high and low gear positions is sometimes provided between a clutch and a main transmission. One example of such splitter is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 8-159258.
The conventional splitter has, however, only two gear positions, i.e., high and low, for high and low speed driving, and is always engaged in one of them.
Some of long-distance drivers have a rest and take a nap in a vehicle, with an air conditioner being turned on. Such vehicle is parked in an idling condition so that a drive power of an engine is transmitted to a counter shaft of the main transmission via the splitter. As a result, a counter gear mounted on the counter shaft and a main gear mounted on a main shaft always engage with each other and rotate. As teeth of the gears encounter, they rattle. The driver inside the vehicle hears this noise.
To overcome this problem, one with an ordinary skill in the art might modify a damping characteristic of a clutch, or additionally provide a scissors gear. In either measures, however, the engine drive power is still transmitted to the counter shaft. Thus, the fundamental problem remains unsolved. Further, a total power transmission line is elongated so that installing a modified clutch/scissors gear itself may be difficult.
An object of the present invention is to overcome the above described problems.
According to one aspect of the present invention, there is provided a multi-stage transmission for a vehicle that includes a main transmission, and a splitter (secondary transmission) located between a clutch and main transmission, wherein the splitter has high and low gear positions and a neutral position. Since a drive power from an engine can be discontinued at the splitter when the splitter is in the neutral position, it is possible to prevent gears from rattling in the main transmission.
The multi-stage transmission may further include a splitter controller for shifting the splitter to the neutral position and maintaining the neutral condition for a predetermined period when predetermined conditions are met. The splitter controller may also cause the splitter to be shifted to the high or low gear repeatedly at predetermined intervals. The splitter controller may determine at least one of the predetermined period and interval from temperature of lubrication oil. A counter shaft in the main transmission is rotated by the engine drive power transmitted via the splitter. The splitter controller may estimate the lubrication oil temperature based on drop of rotation speed of the counter shaft.